The most important parameter when choosing an ESD diode to protect a circuit is the reverse standoff voltage. It can be either only positive or negative or both. In the first case, only undirectional protection is needed since the voltage of the data signal will always be above or always below ground. Therefore, the protection diode is always in reverse blocking mode during normal operation. This is the case for the most common dataline standards, such as USB, HDMI and so on.